Contoured glow discharge cathode producing focused electron beams



J. W. DAVIS Feb. 25, 1969 CONTOURED GLOW DISCHARGE CATHODE PRODUCING FOCUSED ELECTRON BEAMS Filed Aug. 21. 1967 Sheet 1 or 5 INVENTOR. JACK W. DAVIS ATTORNEY .1- W. DAVIS Feb. 25, 1969 CONTOURED GLOW DISCHARGE CATHODE PRODUCING FOCUSED ELECTRON BEANS Shet Filed Aug. 21. 1967 ARI n I I I n w INVENTOR. JACK W. DAVIS Feb. 25, 1969 J. w. DAVIS 3,430,091

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INVENTOR JACK W. DAVIS United States Patent 20 Claims ABSTRACT OF THE DISCLOSURE A contoured cathode operating in a glow discharge for producing a beam of electrons is described. Improved efficiency, as well as better control of power and depth of field of the cathode is obtained with a shield selectively spaced from the cathode and provided with a controllable aperture. Shaping of the beam in a nondissipative manner is shown.

This application is a continuation-in-part of my application entitled, Contoured Cathode, Ser. No. 508,314 filed Nov. 17, 1965, now abandoned.

Background of the invention Conventional means for producing electron beams involve the liberation of electrons from the surface of a heated cathode by thermionic emission. Recently, electron beams have been produced from a narrow apertured cold hollow cathode as a result of the volume production of electrons within a hollow chamber enclosed by the cathode. In glow discharge devices various operating modes may be encountered if one for instance varies the potential difference between the cathode and the anode and the gas density of the environment. One of these modes produces a well-defined electron beam which may predictably and advantageously be used to work materials. Other modes such as the arc mode also are a good source of electrons but this mode exhibits erratic behavior and does not produce a high energy beam.

These narrow apertured hollow cathodes have hollow chambers fabricated from a wire mesh or solid metal with a single aperture in one end. When the cathode is subjected to a high negative potential with respect to an anode and with the proper cathode geometry and pressure level in the hollow chamber, a well-defined pencil beam of high current density, high energy electrons emanates from the aperture. An example of the versatility of the configurations possible with the apertured hollow cathode may be found in US. Patent No. 3,381,157.

The hollow cathode has a cathode fall region adjacent its external surface The electrons for the beam are obtained, however, from a plasma generated by an intense discharge within the chamber enclosed by the cathode. If the aperture through which the electron beam emerges is then'so shaped to where at least one cross-sectional dimension is less than the size of the cathode fall region,

an electrostatic focussing effect on the electrons is obtained and a well-defined beam is produced.

The range of gas densities and voltages in which this narrow apertured hollow cathode operates with stability with little tendency to enter the arc mode is generally small. For instance, an apertured cathode operating in argon at 24 microns of mercury pressure must operate at 1200 volts potential and will enter the arc mode if the potential is raised to 2000 volts or the pressure is increased to 40 microns. At about 7 microns of pressure, a potential of 10,000 volts can be used but an increase in the pressure to 10 microns will result in an arc mode.

Hence, with each particular gas density necessary for an electron beam mode, there is a maximum voltage atwhich a relatively small increase in gas density will shift the operation into the arc mode.

A particular operating regime cannot be easily defined for the narrow apertured hollow cathode since this depends upon many factors such as, type of gas used, the density of the gas operating voltage, geometric shape of the cathode, beam power requirements, stability, and where the anode is close to the cathode the cathode-toanode spacing, etc. Nevertheless, it can be stated with reasonable accuracy that its operation for most useful applications involves gas pressures of about 50 microns of mercury or less. For high voltage applications of greater than 20 kv., much lower pressure regimes of the order of 10 microns or less are required and are subject to instability with relatively small excursions in the gas pressure due to out gassing or other factors. Narrow apertured hollow cathodes have been operated in the electron beam mode at high pressures but their dimensions are then so small as to seriously limit the power levels below practical utility for most electron beam applications.

The many useful functions to which electron beams may be put are illustrated for instance by the patent to Steigerwald, No. 2,987,610, directed at deeppenetration of materials and Patent No. 2,902,583 directed at pulsed I electron beam working of materials. To achieve the deep penetration, the power density of the beam at the workpiece must be sufficiently high. In the event of conventional welding, a surface electron beam working technique is used, sufiicient power is needed to heat and melt an area on the surface of the workpiece with subsequent fusion at deeper levels in the material occurring by the conduction of the heat from the surface.

As a result of working of materials with an electron beam, a substantial amount of out gassing and evaporation from the workpiece occurs. This causes excursions from a mean pressure level of the gas and increases, the probability of arcing in the apertured hollow cathode. Furthermore, temperature variations of the gas cause excursions in its density and further reduce the stability of the operation. In practical applications these factors have made high voltage operation of about 15,000 volts highly unstable with an increase in the, pressure of several microns causing an arc mode and a comparable decrease causing extinguishment of the discharge. In fact complicated and expensive closed loop gas pressure controls have been found necessary to maintain the electron beam from an apertured hollow cathode. These disadvantages have been overcome with a newly developed glow discharge cathode.

Summary of invention Accordingly, it is an object of this invention to provide a glow discharge device for producing an electron beam at high pressure levels.

It is a further object of this invention to provide a cold glow discharge device operating in a gaseous environment for producing an electron beam and the stability of which is insensitive to excursions in the density of the gas.

It is a further object of this invention to toured glow discharge cathode.

It is still another object of this invention to provide a shielded contoured glow discharge cathode for focussing the electrons to a high degree.

It is another object of this invention to provide a contoured cathode having a high efliciency.

It is still further an object of this invention to provide a shielded surface emitting cathode operating in a glow provide a condischarge mode for providing a beam of electrons having a shape and magnitude commensurate with the aperture provided in the shield located opposite the emitting surface of the cathode.

Description of the preferred embodiment A glow discharge cathode has been developed which produces a highly focused electron beam and also operates in a high pressure region. This newly developed glow discharge cathode produces the electrons for the beam with a different mechanism for that relied upon in the hollow cathode. The new cathode produces the electrons from secondary emission processes such as from ions striking its surface and is therefore a surface emitter. Focusing is accomplished by contouring the surface of the cathode.

In the glow discharge mode practically all of the potential drop from cathode to anode occurs across a small cathode fall region adjacent the cathode. Since equipotential lines near the cathode follow the contoured surface strong electrostatic focusing of the electrons emitted from the cathode surface may be accomplished. By contouring the surface of the cathode along a constant radius, by majority of the electrons will be focused at the center of curvature. For constant radii cathodes the electrons are focussed at a center of curvature which lies outside the cathode fall.

The efiiciency of the contoured cathode can also be substantially imhproved by the use of a shield surrounding the cathode. These shields may be made of a conductor material or an insulator such as disclosed in the application U.S. Ser. No 506,237, filed Nov. 3, 1965 by Conrad M. Banas and Clyde 0. Brown entitled, Insulator Shielded Cathode, now abandoned. The shield suppresses the emission of electrons from those surfaces on the cathode where the electrons would not contribute to the main beam. The contoured cathode permits the design of a shield of unusual effectiveness in inhibiting the formation of a plasma in the gap between the shield and the cathode.

FIGURE 1 shows the formation of an electron beam with a contoured cathode.

FIGURE 2 shows an annular contoured cathode with a shield.

FIGURE 3 shows the mounting arrangement for the annular contoured cathode of FIGURE 2.

FIGURE 4 shows a cylindrically shaped shielded contoured cathode.

FIGURE 5 shows a hemispherically shaped shielded contoured cathode.

FIGURE 6 shows a variable aperture configuration of a shielded contoured cathode.

FIGURE 7 shows a nondissipative mask for a planar cathode.

In FIGURE 1 the cathode 1 and anode 3 are placed in a chamber 2 which is evacuated to the desired pressure region to establish a glow discharge. As can be seen, the equipoteritial lines near the cathode 1 follow the contour of the cathode 1 but away from the cathode they are no longer curved. Since the cathode fall occurs close to the cathode surface, the acceleration of the electrons occurs through a region where the shaping of the cathode can strongly influence the focussing of the beam generated from the surface of the cathode 1. Electrons travel along the field intensity lines 5. These field lines 5 are quite Weak outside the cathode fall region and influence the electron paths very little. Hence, by contouring the cathode 1 along a constant radius of curvature the electrons may be made to converge along a straight path like 4 towards 'a focal point which is the center of curvature of the cathode surface.

Since the contoured cathode does not enclose a hollow environment, it does not produce the electrons as a function of an enclosed volume. Instead electrons are obtained by the secondary emission effects upon the cathode surface. For this reason a good producer of electrons will be a cathode having a high secondary emission coeflicient.

Although a coolant feature is not shown in the drawings, the operation of the contoured cathode can be improved by appropriately cooling the cathode. Cooling is especially desirable, and often essential, for operation at high power density levels. Although cooling can be provided by many different means, water-cooling is one method that has been used successfully. Cooling the cathode results in improved operation because, for many cathode materials, the secondary emission coefficient due to ion bombardment increases with decreasing temperature. Thus cooling the cathode results in an increase in power efliciency. Also by cooling the cathode and thereby maintaining the surface temperature below thermionic emitting temperature, the occurrence of the arc mode of operation is minimized. Further, by cooling the cathode and associated structure, thermal expansion and attendant warping and misalignment of these components are mini mized, thus maintaining structural integrity.

The contoured cathode must achieve certain minimum dimensions in order to produce the focussing effect. Thus, with a cross-sectional view of the cathode as in FIGURE 1, the total effective height dimension A must be larger than the cathode fall distance D to assure good focussing. If A is too small, the cathode will not sufficiently perturb the equipotential lines throughout the cathode fall region to achieve the focussing effect. The cathode 1 focusses the electrons at a line which is the center of curvature of the contoured surface. The line is located in a plane that bisects the arc extended by the surface and is perpendicular thereto. The total effective height dimension A in the cathode 1 is measured along the chord formed by the line connecting the ends of the arc and perpendicular to the plane. As will hereinafter be described, the effective height of the cathode may be somewhat less than the chord length where the cathode shield protrudes over the frontal surface.

Stability of the electron beam produced by the contoured cathode with the shield is obtained over a wide range of voltages with pressures as high as 1,000 microns of mercury. For instance, it has been possible to butt weld two four-inch outside diameter stainless steel tubes having a Az-inch wall with an annular contoured shielded cathode having an eight-inch inside diameter with a twoinch radius of curvature. The potential was 8,000 volts and the pressure 200 microns of mercury.

FIGURE 2 shows an annular embodiment of the contoured cathode. The shield 12 is shown mounted about the outside surface of the cathode 14. FIGURE 3 shows how the cathode is supported by carefully shaped pins at various places around the perimeter of the shield. The supporting arrangement is more specifically described in the copending application U.S. Ser. No. 508,201, filed Nov. 17, 1965, entitled, Mounting for a Glow Discharge Cathode, by Allan P. Walch and assigned to the same assignee. The shield may be supported by the wall through the cantilever support provided by the cathode conductor feedthrough assembly 16.

The shield 12 extends over the contoured cathode 14 with a lip-like extension 18. The extension 18 shields a portion of-the contoured cathode within its vicinity. The gap 20 that separates the shield 12 from the coutoured cathode 14 is maintained all over the outer surface of the cathode except where it is supported by pins and where the biasing conductor 24 requires connection to the cathode. The extension 18 shields the gap 20 from particles traveling towards the cathode surface and thereby more efficiently suppresses the formation of a plasma therein and enhances the stability of the device. Since the extension suppresses in its vicinity the emission from the cathode surface the effective height A is reduced and a larger height may be needed to compensate for the loss of emitting surface area.

Other configurations can be envisioned; for instance, the hemispherically-shaped shielded contoured cathode of FIGURE 4. This cathode produces a beam of electrons focused to a small spot.

FIGURE 5 shows a straight, cylindrically-shaped shielded cathode where the beam of electrons is focused on the cylindrical axis.

Other configurations are possible such as where the annular contoured cathode radiates outwardly instead of inwardly as shown in FIGURE 2 for use in welding the inside of a pipe.

The radius of curvature may be varied substantially to the extent where the radius is so large that the cathode assumes, for practical purposes, a straight disc. The length the focal distance may be is to be determined by practical considerations involved with the working of materials with a beam of electrons. For instance, for the welding of materials according to the invention disclosed in the patent to Steigerwald, No. 2,987,610, the radius of curvature in combination with the area of the cathode must be so established that the beam power density at the focal point or at the point where the beam operates upon the workpiece is sufliciently high to achieve the deep penetrations. In the event surface welding techniques are to be relied upon, the area of the contoured cathode together with the radius of curvature must be such as to produce sufiicient heat at the workpiece.

The extension of a lip 18 over the frontal emitting surface of the cathode may be used to advantageously control the shape and power of the beam in a nondissipative manner. FIG. 6 discloses such a device with a moveable shield.

A cathode 30 having a straight linear shape is provided with a curved frontal surface 31. This frontal surface 31 is curved along one direction to focus the beam of electrons in a line generally indicated at 32. Surrounding the cathode in the back and the top and bottom sides is a shield 33 which extends all along both side of the cathode 30. The shield 33 is selectively spaced from the cathode 30 to properly suppress the glow discharge in the gap.

In addition, a movable shield section having an upper portion 34 and a lower portion 35 are provided. The upper portion 34 is hingeably connected to the shield 33 along the top line 36 of the shield 33 and the lower portion 35 is hingeably connected to the shield 33 along the bottom line 37. Each of the portions are provided with lip sections 38 and 39 respectively and as the portions are rotated to bring the lip sections over the frontal surface 31 the glow discharge between the lip portions and the cathode 30 is extinguished and prevented from forming.

By varying the position of the upper and lower portions of the shield the exposed emitting area of the cathode 30 can be varied, in other words, the shield masks out portions of the cathode and acts as a variable aperture. The spacing between the lip portions 38 or 39 and the frontal surface is typically of an inch to A; of an inch and is much less than the normal cathode fall distance of the discharge so that a discharge does not occur between the cathode and the lip portions. Thus the lip portion or mask sections of the cathode does not emit electrons which would otherwise be accelerated into the mask thereby providing a nondissipative mask. Although no significant difference in the effectiveness of conducting or insulating masks has been observed to date, the mask material and its shape has some effect on the focusing characteristics of the cathode, especially in the immediate vicinity of the mask. This effect is due to perturbations of the equipotential lines by the mask. These perturbations are evidenced by small aberrations in the electron beam, similar to aberrations that sometimes occur in optical lenses due to imperfections in grinding near the edges. The aberrations can be minimized by designing the shields in such a manner as to minimize their effect on the equipotential lines. In other cases, however, it may be quite desirable to utilize these perturbation effects to achieve some electrostatic focusing of the electron beam.

Again, the actual shield design would be dictated by the type of focusing required but this could be guided by standard equipotential line mapping techniques.

FIG. 6A shows the partial closing of the movable mask or lip portions of the shield sections 34 and 35. The focus of the electron beam emitted by the cathode 30 is independent of the aperture size, yet the aperture size may affect the depth of field of the electron beam and thus control that particular parameter. In addition, the size of the frontal surface exposed to the glow discharge will affect the total beam power by limiting the current and thus this is also controllable by varying the position of the movable lips 34 and 35.

The lip or mask portion of the shield may be provided with any arbitrary geometry to obtain the desired shaping of the beam. In some instances when the workpieceis irregular in shape, a similarly shaped beam may be easily obtained with a mask having an aperture shaped like the workpiece. In such a case a mask 42 of the type shown in FIGURE 7 may be employed. In FIGURE 7 a planar cathode 43 is shown to produce a beam of electrons that is commensurate with this shape of the aperture cut into the shield 42. The aperture 44 is irregularly shaped and the beam has a corresponding irregular pattern. The nondissipative masking of the beam permits high powered application with a large efficiency not possible in previous devices. The particular mask 44 is especially useful for surface processes such as electron beam hardening of oddly shaped workpieces and for thin film work and eliminates complicated scanning patterns for pencil-type electron beams. In this respect such a nondissipative masked electron beam is especially attractive for high production rate processes.

In addition, multiple beams may be generated from such a nondissipative mask by providing multiple apertures 44 in a mask.

It is to be understood that the invention is not limited to the specific embodiment herein illustrated and described but may be used in other ways without departure from its spirit as defined by the following claims.

Having thus described my invention, What I claim is:

1. A cathode structure operating at a high negative potential with respect to an anode in a gaseous chamber evacuated to a predetermined pressure range of the gas comprising:

means for establishing a glow discharge and a cathode fall for the production of an electron beam from the cathode structure,

said cathode structure being contoured and having a frontal surface exposed to the gas in the chamber for the emission of electrons from the frontal surface, and

where the electrons from said cathode frontal surface are emitted from an area having a radius of curvature greater than the cathode fall distance.

2. A device as recited in claim 1 where said cathode structure is further surrounded with a shield selectively spaced from said cathode, and

said shield having an opening located opposite to and similarly shaped as said electron emitting area of said cathode frontal surface.

3. A device as recited in claim 2 Where said shield has an extension over said cathode frontal surface for shielding the space between the cathode structure and the shield from ions traveling towards the cathode.

4. A device as recited in claim 1 where said cathode frontal surface is contoured along a constant radius of curvature for focusing of the electrons at the center of curvature.

5. A cathode structure operating at a high negative potential with respect to an anode in a gaseous chamber evacuated to a predetermined pressure range of the gas comprising:

means for establishing a glow discharge and a cathode fall in said chamber for the production of an electron beam from a surface of the cathode structure,

said cathode structure having said surface exposed to the gas in the chamber for the emission of electrons therefrom,

said surface defining a contoured concave section for focusing of the emitted electrons in a line located in a plane substantially perpendicular to and bisecting the contoured section, and

Where the chord connecting the edges of the contoured section and substantially perpendicular to said plane has a length greater than the cathode fall distance.

6. A device as recited in claim where the cathode structure is provided with a shield selectively spaced therefrom and having an opening opposite said cathode surface,

said opening having a similar shape as said cathode surface.

7. A device as recited in claim 6'where said section is contoured along a constant radius of curvature.

8. A device as recited in claim 6 where the radius of curvature is greater than the cathode fall distance.

9. A device as recited in claim 6 where said shield is further provided with an extension over the electron emitting surface of the cathode structure for shielding the space between the cathode structure and the shield from ions traveling towards the cathode.

10. A cathode structure operating as a high negative potential with respect to an anode in a gaseous chamber evacuated to a predetermined pressure range of the gas comprising:

means for establishing a glow discharge and a cathode fall in said chamber for the production of an electron beam from a surface of the cathode structure,

said cathode structure having said surface exposed to the gas in the chamber for the emission of electrons therefrom, and

said cathode surface forming a concave spherical section in which the chord connecting the edges of the concave spherical section is greater than the cathode fall for focusing of the electrons at a focal point commensurate with the center of curvature of the spherical section.

11. A device as recited in claim 10 wherein the cathode structure is further provided with a surrounding shield selectively spaced therefrom and having an opening located opposite to and similarly shaped as the spherical section.

12. A device as recited in claim 11 wherein the shield is further provided with an extension over the spherical section and the effective maximum chord length is greater than the cathode fall.

13. An annular cathode structure operating at a high negative potential with respect to an anode in a gaseous chamber evacuated to a predetermined pressure range comprising:

means for establishing a glow discharge and a cathode fall for the production of an electron beam from the cathode structure,

said cathode structure having a concave annular surface with annular edges and contoured to focus the electrons at the center of curvature of the contoured surface,

a shield selectively spaced from and surrounding the cathode structure and provided with an opening of similar shape as the concave surface, and

said shield having an extension to cover a portion of said concave surface adjacent said annular edges and where the maximum effective chord length of the concave surface is greater than the cathode fall.

14. A cathode for the emission of a beam of electrons in a glow discharge comprising:

a cathode having a nonemitting surface and a concave emitting surface,

said emitting surface terminating in at least one edge,

and

a shield opposite said nonemitting surface and spaced therefrom and including a lip surrounding said edge and extending to a point adjacent said emitting surface.

15. A cathode for the emission of a beam of electrons in a glow discharge comprising:

a nonemitting surface and a concave emitting surface,

said emitting surface being contoured and elongated in at least one dimension, and

a shield opposite said nonemitting surface and spaced therefrom to suppress the formation of said glow discharge in the space between the nonemitting surface and said shield.

16. A cathode according to claim 15 wherein:

said shield extends with a lip opposite said emitting surface to prevent ions travelling towards the cathode from entering the space between the cathode and the shield.

17. A cathode structure operating at a high negative potential with respect to an anode in a gaseous chamber evacuated to a predetermined pressure range of the gas comprising:

means for establishing a glow discharge and a cathode fall for the production of an electron beam from the cathode structure, said cathode structure having a frontal surface exposed to the glow discharge in the chamber for the emission of a beam of electrons from the frontal surface,

said cathode structure being surrounded with a shield selectively spaced from said cathode,

said shield further comprising:

a nondissipative masking section selectively spaced opposite said frontal surface and masking predetermined areas of the frontal surface from particles in the glow discharge,

said section further being provided with an aperture having a preselected shape commensurate with the desired shape of the beam of electrons.

18. A device as recited in claim 17 wherein said nondissipative masking section comprises:

a lip portion partially extending from said shield over said frontal surface for suppressing the formation of a plasma between the lip and the frontal surface.

19. A device as recited in claim 17 and further comprising:

means for varying the power in the beam of electrons and including means for varying the size of the aperture in the nondissipative masking section.

20. A device as recited in claim 17 and further comprising:

means for varying the depth of field of the beam of electrons and including means for varying the size of the aperture in the nondissipative masking section.

References Cited UNITED STATES PATENTS 2,038,825 4/1936 Canady 313217 X 2,056,662 10/1936 Foulke 313-410 X 2,116,672 5/1938 Ewest et al. 313-210 X 2,695,970 11/1954 Nazzewski 313210 X JOHN W. HUCKERT, Primary Examiner.

A. I. JAMES, Assistant Examiner.

U.S. Cl. X.R. 

